1. Field of the Invention
The present invention relates generally to methods for fabricating II-VI semiconductor electronic devices. In particular, the present invention is a method for fabricating an interface between layers of III-V semiconductor and II-VI semiconductor.
2. Description of the Related Art
Electronic devices made from ZnSe, MgZnSSe and other II-VI semiconductors are generally known. By way of example, laser diodes fabricated by molecular beam epitaxy (MBE) from these and other II-VI semiconductor compounds are disclosed in the Haase et al. U.S. Pat. No. 5,291,507 and the Cheng et al. U.S. Pat. No. 5,319,219. These devices are typically fabricated on substrates of GaAs or other III-V semiconductor compounds.
Unfortunately, defects known as stacking faults form at or near the interface of the III-V and II-VI semiconductors during MBE growth. Techniques for measuring stacking fault densities in III-V/II-VI interfaces are known, and disclosed in the Kamata et al. article, Characterization of ZnSSe on GaAs by Etching and X-ray Diffraction, J. Crystal Growth, Vol. 142, pp. 31-36 (1994). It is very important to minimize the presence of these stacking faults since they act as non-radiative recombination centers, dissipate recombination energy, generate further defects and thereby shorten the operative lifetime of the devices.
Known methods for growing II-VI semiconductors on III-V substrates, and for preparing the substrates for II-VI semiconductor growth, are disclosed in the following references:
1. S. Guha et al., Structural Quality and the Growth Mode in Epitaxial ZnSe/GaAs(100), J. Appl. Phys., Vol. 73, No. 5, pp. 2294-2300 (March 1993). PA1 2. J. Gaines et al., Structural Properties of ZnSe Films Grown by Migration Enhanced Epitaxy, J. Appl. Phys., Vol. 73, No. 5, pp. 2294-2300 (March 1993). PA1 3. D. Li et al., ZnSe Nucleation on the GaAs(001):Se-(2.times.1) Surface Observed by Scanning Tunneling Microscopy, J. Vac. Sci Technol. B. (July/August 1994). PA1 4. Cheng et al., Molecular-Beam Epitaxy Growth of ZnSe Using a Cracked Selenium Source, J. Vac. Sci. Technol. B., Vol. 8, No. 2 (March/April 1990). PA1 5. The Technology and Physics of Molecular Beam Epitaxy, edited by E. H. C. Parker, Plenum Press, 1985.
In general, one approach involves nucleating ZnSe on a bare GaAs substrate that has been heated (either in the II-VI growth chamber or a separate ultra-high-vacuum chamber) to desorb native oxide. However, this approach presents a number of problems. Thermally treating the GaAs surface tends to increase surface roughness, and the possibility of incomplete substrate deoxidation remains. The stoichiometry of the starting GaAs surface is difficult to control by this process. Contamination of the GaAs surface by the group VI elements in the II-VI growth chamber has been observed. It is also difficult to reproducibly obtain high-quality interfaces by this method
Another approach involves desorbing the GaAs substrate in a separate GaAs growth chamber under an As flux (to reduce Ga evaporation and surface roughening). A GaAs buffer layer is grown on the substrate to obtain a smooth GaAs surface. The wafer is then cooled, transferred in-vacuo to a II-VI growth chamber, and heated to approximately 300.degree. C. for ZnSe nucleation. Although an improvement over the technique described above, this growth method still suffers from a number of disadvantages, such as increased contamination due to cooling of the GaAs buffer layer and transfer through the ultra high vacuum pipeline, and contamination by the group VI source. For these and other reasons, a sufficient level of reproducibility has not been demonstrated.
In general, it is difficult to reproducibly fabricate III-V/II-VI interfaces with stacking fault densities less than about 1-5.times.10.sup.6 /cm.sup.2 using these known techniques. The commercial viability of laser diodes and other II-VI semiconductor devices would be greatly enhanced by methods enabling these devices to be reproducibly fabricated with lower stacking fault densities.